Coastal Construction Manual - National Ready Mixed Concrete ...

6 FUNDAMENTALS OF RISK ANALYSIS AND RISK REDUCTION
a danger to the public and buildings associated with utilities required to maintain the use of other
buildings in this category.
Performance expectations for buildings vary widely depending on the type of hazard being resisted.
Selection of the design event in the I-Codes is determined by the hazard type, the risk category of building,
and the type of building damage expected. Selecting a higher risk category for most residential buildings
should result in a higher final design wind pressure for design and should improve building performance in
high-wind events. It can also result in additional freeboard in Zone V and Coastal A Zone if using ASCE
24 in flood design.
For flood hazard design, the building is divided into two
distinct parts: the foundation and the main structure. For the
foundation, standard methods of design target an essentially
elastic response of the foundation for the design event such that
little or no structural damage is expected. The main structure
is designed to be constructed above the DFE to eliminate the
need for designing it to resist flood loads. If flooding occurs
at an elevation higher than the DFE, flood loads can be
significant where flood waters impact solid walls (as opposed
to open foundation elements). Additionally, a water level only
a few inches above the minimum floor elevation can result in
damage to walls and floors, and the loss of floor insulation,
wiring, and ductwork. The IRC incorporates freeboard
for houses in Zone V and Coastal A Zone, and the IBC
NOTE
Designing to only minimum code
and regulatory requirements
may result in designs based on
different levels of risk for different
hazards. The importance of each
hazard level addressed by such
requirements, and whether an
acceptable level of residual risk
remains, should therefore be
carefully considered during the
design process.
incorporates freeboard for buildings by virtue of using ASCE 24. Including freeboard in the building design
provides a safety factor against damage to the main structure and its contents caused by flood elevations
in excess of the design flood. While codes and standards set minimum freeboard requirements, a risk
assessment may indicate the merits of incorporating additional freeboard above the minimum requirements
(see Sections 6.2.1.3 and 6.3).
For wind hazard design, standard methods of design also target an essentially elastic response of the
building structure for the design event (i.e., 700-year wind speed, 3-second gust per ASCE 7-10) such that
little or no structural damage is expected. For wind speeds in excess of the design event, wind pressures
increase predictably with wind velocity, and factors of safety associated with material resistances provide a
margin against structural failure.
For seismic hazard design, life safety of the occupants is
the primary focus rather than preventing any damage to the
building. All portions of the building should be designed to
resist the earthquake loads. Buildings are designed using the
Maximum Considered Earthquake (i.e., 1 percent in 50 years)
and include factors such as ground motion and peak ground
acceleration. Adjustment factors are applied to design criteria
based on the risk category for the building.
For erosion hazard design for bluff-top buildings, the ratio
of soil strength to soil stresses is commonly used as the safety
factor by geotechnical engineers when determining the risk of
NOTE
In the past, little thought was
given to mitigation. Homeowners
relied on insurance for
replacement costs when a natural
hazard event occurred, without
regard to the inconvenience and
disruption of their daily lives.
Taking a mitigation approach can
reduce these disruptions and
inconveniences.
6-8 COASTAL CONSTRUCTION MANUAL